The present invention relates to the formation of grating structures in optical media and, more particularly, to the incorporation of a feedback mechanism to control the determination of specified grating characteristics during the grating writing process.
Optical waveguide gratings, such as fiber Bragg gratings, are recognized as key components for many optical communication systems. In particular, a Bragg diffraction grating is a structure that has a periodic pattern of alternating high and low optical refractive index values. Bragg gratings are useful as a result of their ability to reflect a particular wavelength or xe2x80x9ccolorxe2x80x9d of light. The color that will be reflected by a grating is the color whose wavelength exactly matches twice the effective grating period.
It is well known that Bragg gratings may be formed by using an external source of optical radiation to create an interference pattern in the germanosilicate glass core of an optical fiber (or in any other suitable optical medium, such as an optical waveguiding substrate). In the bright sections of the interference pattern (where the interfering beams reinforce each other), the beams interact with germanium sites in the fiber core (waveguide) and change the value of the refractive index. In the dark sections of the interference pattern (where the interfering beams destructively interfere and cancel each other), the core refractive index will remain unchanged. Thus, the interference pattern creates a regular, periodic change in refractive index along a section of the core, forming a Bragg grating.
To provide greater flexibility in the design of fiber optic Bragg grating devices, techniques have been developed to write gratings by applying the optical radiation through the side of an optical fiber. One such technique, as illustrated in U.S. Pat. Nos. 4,725,110 and 4,807,950, involves splitting a laser beam into two sub-beams at a known and controllable angle within the core of the optical fiber (thus forming the interfering beams as discussed above). A second well-known technique described in the technical and patent literature involves focusing the laser beam on the fiber core through a grooved or patterned transmissive optical element referred to as a xe2x80x9cphase maskxe2x80x9d. The phase mask holographically creates the required interference pattern in the fiber core.
The above-described techniques for producing optical fiber Bragg gratings are well established, but certain technical difficulties have prevented their use in large scale continuous or stepwise continuous production processes. U.S. Pat. No. 6,072,926 issued to M. Cole et al. on Jun. 6, 2000, discloses a method of writing gratings in an optical medium where both the medium and the phase mask are moved with respect to one another during the writing process, so as to vary the grating properties along the length of the grating, allowing for a relatively long, continuous grating to be formed. Relative movement in a single direction with this technique can result in a change in the grating pitch and, therefore, can be used to fabricate chirped or multi-wavelength gratings. Cole et al. also teaches the application of bi-directional dither to the fiber during the writing process to fabricate an apodized grating. xe2x80x9cApodizationxe2x80x9d is a technique of modifying the envelope of (in this case) the grating pattern to reduce the presence of the side lobes on either side of the main lobe in the pattern. When the grating is used as an optical filter (e.g., in a wavelength division multiplexed (WDM) system) the spacing of the grating pattern may be apodized such that the main lobe corresponds to a particular center wavelength, thus reducing the presence of optical crosstalk between channels. The apodization technique utilizes UV laser beams of relatively small width and is referred to in the art as xe2x80x9cpoint-by-pointxe2x80x9d writing.
Phase and amplitude mask fabrication methods are also well-known in the art and include the UV exposure or holographic technique, electron beam writing technique, mechanical deformation, and others. See, for example, Diffraction Gratings and Applications, by E. G. Loewen et al., 1997.
While these and various other techniques continue to be developed and perfected to generate ever more complicated grating patterns, the accuracy and repeatability of such grating fabrication processes, as required for a high throughput manufacturing environment, remains problematic. Thus, a need remains in the art for a technique to improve the quality (in terms of accuracy and repeatability) of the various types of gratings written in optical media.
The need remaining in the prior art is addressed by the present invention, which relates to the formation of grating structures in optical media and, more particularly, to the incorporation of a feedback mechanism to control the determination of specified grating characteristics during the grating writing process.
In accordance with the present invention, a known grating characteristic (for example, grating period chirp, reflectivity and/or group delay) is measured during the writing process. The xe2x80x9cerrorxe2x80x9d between the expected result and the actual measured characteristic is determined using numerical algorithms known in the art of grating characterization and used as a feedback signal to the writing process. The feedback introduces a xe2x80x9ccorrectionxe2x80x9d step in the writing process, to be used either during the current writing step, or as a xe2x80x9cpost-writingxe2x80x9d correction process, to modify the written grating characteristic so as to more closely match the expected result.
In its most general terms, the corrective feedback technique of the present invention may be applied to the actual grating formed in the optical medium, or (if present) to the diffraction mask (phase or amplitude, for example) used to create the grating. Advantageously, if an error in the mask is discovered and xe2x80x9ccorrectedxe2x80x9d, the remaining gratings formed using that mask will be free of the particular defect associated with the error (as indicated by, for example, the magnitude of the group delay ripple). The process of the present invention may be iterative, with multiple measurements and error signals generated in succession, until an optimum grating structure is achieved. This aspect is particularly well-suited for applications where it is desired to form a grating with tightly-controlled parameters. For example, the inventive corrective process may be used to continuously monitor and adjust the characteristics of a chirped grating to achieve a grating yielding a dispersion factor D on the order of 800 ps/nm, with a chirp linearity of better than 1 ps.
In one embodiment of the present invention, the xe2x80x9cerrorxe2x80x9d measured in a written grating may be used to correct the writing process, where the actual grating that exhibited the error is not corrected, but rather is discarded as a xe2x80x9ctestxe2x80x9d grating, that is, the grating used to determine the error between the parameters of the actual grating and the desired parameters.
In a specific embodiment of the present invention, the corrective feedback technique may be used to correct the group delay ripple associated with chirped fiber gratings. In this embodiment, the group delay characteristics are measured and the xe2x80x9cripplexe2x80x9d is used to generate the correction for the DC and/or AC refractive index profile, defined as the corrective refractive index profile, or simply the corrective profile. A further, non-uniform UV exposure, non-uniform annealing, non-uniform heating and/or non-uniform applied tension, applied separately or in an intermittent sequence, can be used to a corrective profile.
Other and further aspects and embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.